Bath for the electroless deposition of palladium



United States Patent 3,418,143 BATH FOR THE ELECTROLESS DEPOSITION OF PALLADIUM Alexander Sergienko, La Puente, Calif., assignor to Burroughs Corporation, Detroit, Mich., a corporation of Michigan No Drawing. Continuation of application Ser. No. 391,727, Aug. 24, 1964. This application Aug. 15, 1967, Ser. No. 660,588

22 Claims. (Cl. 106-1) ABSTRACT OF THE DISCLOSURE A bath for the electroless deposition of palladium comprising an aqueous solution of divalent palladium, ethylenediarninetetraacetic acid, or a salt of that acid, ethylenediamine, and hypophosphite ion. The ethylenediamine and ethylenediaminetetraacetic acid, or a salt of that acid, form chelates with the divalent palladium in the plating solution to thereby provide an exceptionally stable plating bath.

This application is a continuation of the copending application, now abandoned, bearing Ser. No. 391,727, filed on Aug. 24, 1964, entitled, Bath for the Electroless Deposition of Palladium, and assigned to the same assignee as the present application.

This invention relates to a bath for the electroless deposition of palladium.

Electroless deposition is the process by which metal is reduced from a solution of suitable composition and deposited on a catalytic surface through oxidation of a reducing agent at that surface. Platinum Group metals, gold and some other metals are catalytic to the reaction and surfaces which are not catalytic to the reaction can be rendered so by well known procedures.

Deposition is accomplished by immersing the catalytic surface in the electroless deposition bath for a predetermined time in accordance with the thickness of deposit desired. Since no electric current is employed, no electrical contact to the surface to be plated is required. For a given substrate material, the chemical reduction or the deposition rate can, within limits, be controlled by the composition and temperature of the plating bath.

At the present time, the only technique for electroless deposition of palladium that is known to have been reported is disclosed in Patent No. 2,915,406. However, the electroless plating bath that is described in that patent is chemically unstable and decomposes rapidly. Such a bath is not suitable for many types of plating operations.

The present invention provides an improved bath for the electroless deposition of palladium that not only permits palladium to be readily deposited on metallic and non-metallic substrates but which bath possesses a high degree of chemical stability. The baths of the present invention are not only stable during plating operations but are also sutficiently stable to be stored and reused. In addition, due to the high chemical stabilities of the solutions and the ease with which the baths may be mainice tained, they may be used in plating operations of any and varying magnitude.

The electroless palladium plating solution of the pres ent invention comprises an aqueous solution containing divalent palladium which is chelated with ethylenediaminetetraacetic acid, C H N O or with a salt of that acid and ammonium, lithium, sodium, potassium or calcium or combinations thereof and ethylenediamine, NH CH CH NH hypophosphite ion, H PO as hypophosphorous acid or as the ammonium, lithium, sodium, potassium or calcium salt of that acid. A solution incorporating these ingredents has demonstrated a high order of chemical stability over a wide range of temperatures and when maintained at a suitable temperature and pH performs very well as a bath for the electroless deposition of palladium.

The present invention teaches that the divalent palladium may be introduced into a new bath in the chelated form or that the chelate may be formed in solution by adding a suitable salt of divalent palladium to a solution containing a suitable concentration of ethylenediaminetetraacetic acid or of a salt of that acid and ammonium, lithium, sodium, potassium or calcium or combinations thereof.

It has been established through the use of the bath of the present invention that the chemical stabilities of the bath are such that the palladium and hypophosphite contents of the solutions may be depleted through use and must be replenished. The palladium content may be replenished by addition of a suitable salt of divalent palladium and formation of the chelate in the solution. The hypophosphite content is replenished by addition of hypophosphorous acid or of an ammonium, lithium, sodium, potassium or calcium salt of that acid to the bath after the palladium has been chelated.

These and other features of the present invention may be more fully appreciated in the light of the following specification.

The electroless palladium plating bath of the present invention is an aqueous solution, generally containing the following constituents:

Range of composition,

grams per liter Palladium, Pd, as a chelate of divalent palladium with ethylenediaminetetraacetic acid,

C H N O or with a salt of that acid and ammonium, lithium, sodium, potassium or calcium or with a combination thereof Ethylenediamine, NH CH CH NH Ethylenediaminetetraacetate, C H N O (4), in addition to that present in the palladium chelate 0-36 Hypophosphite ion, H PO 1.810

It should be noted that the pH of the solution may be adjusted upwardly by the addition of ethylenediamine in the range of 9-75 grams per liter or downwardly by the addition of nitric, sulfuric or hydrochloric acid to a desired pH in the range of 8.0l1.0, electrometric.

3 EXAMPLE I Specifically, a bath for electroless deposition of palladium may have as constituents:

Grams per liter Palladium, Pd as a chelate of divalent palladium with ethylenediaminetetraacetic acid, C H N O or with a salt of that acid, with ammonium, lithium, sodium potassium or calcium or with a combination thereof Ethylenediamine, NH CH CH NH Ethylenediaminetetraacetate, (C H N O as ethylenediaminetetraacetic acid or as a salt of that acid and ammonium, lithium, sodium, potassium or calcium or with a combination thereof, and in addition to that contained in the palladium chelate Hypophosphite ion, H PO as hypophosphorous acid, H PO or as the ammonium, lithium, sodium, potassium, or calcium salt of that acid vention will operate over the temperature range of to 190 F. but, for most applications, it has been found that the optimum range of operating temperatures is to 170 F.

An electroless plating bath must be so composed that the reducing agent present in the bath will reduce the plating metal from the solution only at a surface which is catalytic to the oxidation of the reducing agent. Otherwise, the metal is quickly precipitated on the Walls and bottom of the container and the bath is said to have decomposed. In a useful bath, the stability constants of the compounds of the plating metal in solution must be such that decomposition does not occur but also such that reduction of the plating metal at a suitably catalytic surface will proceed at a controlled rate. Complexing, sequestering or chelating agents added to the bath to prevent decomposition but permitting metal deposition are called stabilizing agents. In the baths of the present invention, the stabilizing agents are ethylenediamine and ethylenediaminetetraacetic acid or a salt of that acid with ammonium, lithium, sodium, potassium or calcium or with combinations thereof. Ethylenediamine performs the additional function of increasing the pH of the solution.

It is well known that chelates of many metals with ethylenediamine have lower stability constants than do chelates of the same metals with ethylenediaminetetraacetic acid or with salts of that acid and ammonium, lithium, sodium, potassium or calcium or with combinations thereof. The stability constants of chelates of palladium with ethylenediaminetetraacetic acid or with the aforementioned salts are to great to permit deposition except at a very low rate, and then, only when the temperature of the solution is very high.

On the other hand, it has been determined that, if palladium is chelated only with ethylenediamine, the bath will be useful but the chemical stability of the solution will be decreased and the useful life of the bath will be greatly reduced.

In an electroless plating bath, it is desirable that metallic impurities in the solution be prevented from codepositing with the plating metal. With many metals which may be introduced as impurities in the electroless palladium bath, the stabilizing agents in the baths of the present invention form stable, soluble compounds which minimize codeposition of these impurities with palladium.

Hypophosphite ion, H PO is the reducing agent in the bath of the present invention which when oxidized at the catalytic surface provides the electrons necessary to reduce the palladium from the solution and to deposit it on the catalytic surface. Hypophosphite ion is supplied in the bath as hypophosphorous acid or as an ammonium, lithium, sodium, potassium or calcium salt of that acid.

The following additional specific examples of the invention give a better understanding of the present invention and the advantages thereof.

EXAMPLE II Composition:

Disodium palladium ethylenediaminetetraacetate, io lz z s z 24.2% Pd 24.8 grams per liter. Ethylenediamine,

NH CH CH NH 25.6 grams per liter. Sodium hypophosphite,

NaH PO H PO 4.1 grams per liter. Hydrochloric acid, HCL As required to adjust the pH of the bath to 8.5, electrometric at 20 C.

To properly prepare the above plating bath, the preferred procedure is to add the disodium palladium ethylenediaminetetraacetate, ethylenediamine and sodium hypophosphite to water and dissolve the solids. After adjusting the desired volume, then, adjust the pH of the bath to 8.5, electrometric at 20 C. After heating the bath through a water jacket to a temperature of F. and maintaining the temperature of the bath, begin plating operations.

EXAMPLE III Composition:

Palladium chloride, PdCI 60% Pd. 10.0 grams per liter.

Disodium dihydrogen ethylenediaminetetraacetate,

C H N O Na 19.0 grams per liter. Ethylenediamine,

N-H CH CH NH 25.6 grams per liter. Sodium hypophosphite, NaH PO 73.9% H PO 4.1 grams per liter. Hydrochloric acid, HCl As required to adjust the pH of the bath to 8.5 electrometric at 20 C.

To properly prepare the bath of Example III, add the palladium chloride, ethylenediamine and dissodium ethylenediaminetetraacetate to water and dissolve the solids. Permit the solution to stand until chelation is complete. Chelation can be accomplished in approximately 24 hours if the temperature of the solution is maintained at approximately 160 F. After chelation is complete, cool the solution to 20 C. add the sodium hypophosphite and adjust the pH to 8.5, electrometric at 20 C. Heat the bath through a water jacket to a temperature of 160 F. and while maintaining the temperature of the bath, begin plating operations.

The solutions resulting from the compositions and pro cedures of Examples II and III are essentially identical. In each of these examples, it has been chosen that the electroless plating solution shall contain 6 grams per liter of divalent palladium as disodium palladium ethylenediaminetetraacetate, C H N O Na Pd, 3.0 grams per liter of hypophosphite, H PO as NaH PO and thatthe pH of the bath shall be adjusted to a pH of 8.5, electrometric at 20 C. by addition of hydrochloric acid and that the operating temperature of the bath shall be 160 F. Further, those skilled in the art will recognize that disodium palladium ethylenediaminetetraacetate can be obtained in chrystalline form by reacting equimolal' quantities of divalent palladium and disodium dihydrogen ethylenediaminetetraacetate in aqueous solution and then crystalizing the palladium chelate from the solution. The palladium may be provided in the form of any suitable salt of divalent palladium.

The following chelates of divalent palladium have been found to be usable:

dihydrogen palladium ethylenediaminetetraacetate,

io n a a sodium hydrogen palladium ethylenediaminetetraacetate,

cmH gNgosNapd potassium hydrogen palladium ethylenediaminetetraacetate, CH13N203KPd ammonium hydrogen palladium ethylenediaminetetraacetate, C 'H N O Pd lithium hydrogen palladium ethylenediaminetetraacetate,

C H N o LiPd disodium palladium ethylenediaminetetraacetate,

c oH zNzogNazpd sodium potassium palladium cthylenediaminetetraacetate,

C1 H12N2O NZ1KPd sodium ammonium palladium ethylenediaminetetraacetate, C H N O NaPd sodium lithium palladium ethylenediaminetetraacetate,

c H N o NaLiPd dipotassium palladium ethylenediaminetetraacetate,

potassium ammonium palladium ethylenediaminetetraacetate, C H N O KPd potassium lithium palladium ethylenediaminetetraacetate,

C H N O KLiPd dilithium palladium ethylenediaminetetraacetate,

C H N O Li Pd lithium ammonium palladium ethylenediaminetetraacetate,

C H N O LiPd diammonium palladium ethylenediaminetetraacetate,

C10H20N403Pd calcium palladium ethylenediaminetetraacetate,

c uH zNzogcapd dipalladium ethylenediaminetetraacetate, C H N O Pd Among the purposes for which the baths of the present invention have been employed to deposit palladium on metals are: corrosion protection of electrical contacts, deposition of barrier layers to prevent diffusion between gold films and base metal substrates, rendering passive metals such as aluminum suitable for electrodeposition or electroless deposition of other metals and rendering such surfaces suitable for soldering with tin-lead solder.

These baths have also been used for deposition of palladium or non-metallic substrates, such as ceramics, glass and plastics. Among these uses have been: deposition of electric circuits on ceramic substrates, deposition of bonding layers between nonmetallic substrates and layers of other electrolessly deposited metals and deposition of conductive films on ceramic substrates to serve as carriers for magnetic alloys which were electrodeposited over the palladium. Such magnetic films are employed as recording media in digital computer applications.

What is claimed is:

1. A bath for electroless deposition of palladium comprising an aqueous solution having as constituents:

(a) palladium, Pd, present in a range of from 1.8 to 15 grams per liter, as a chelate of divalent palladium with:

(l) ethylenediaminetetraacetic acid, or an ammonium, lithium, sodium, potassium, or calcium salt thereof, or combined salts thereof, and

(2) ethylenediamine present within a range of from 9.0 to 75 grams per liter;

(b) ethylenediaminetetraacetate in addition to that present in the palladium chelate, within a range of from 0 to 36 grams per liter; and

(c) hypophosphite ion within a range of from about 1.8 to 10 grams per liter, the solution having a pH value within a range of about 8.0 to 11.0, electrometric.

2. A bath for electroless deposition of palladium as defined in claim 1 wherein the hypophosphite ion is provided by hypophosphorous acid, H PO or by an ammonium, lithium, sodium, potassium or calcium salt of that acid, or combined salts thereof.

3. A bath for electroless deposition of palladium as defined in claim 1 containing amounts of ethylenediamine, NH CH CH NH in adition to that required for forming chelate compounds with divalent palladium, for upward adjustment of the pH of the solution.

4. A bath for electroless deposition of palladium as defined in claim 3 additionally containing hydrochloric acid, HCl, nitric acid, HNO or sulfuric acid, H 50 for proper control of the pH of the solution.

5. A bath for electroless deposition of palladium as defined in claim 1 wherein the bath has an operating temperature in the range of to 190 F.

6. A bath for electroless deposition of palladium as defined in claim 5 wherein the bath has an optimum operating temperature in the range of to F.

7. A bath for electroless deposition of palladium as defined in claim 1 containing dihydrogen palladium ethylenediaminetetraacetate, C H N o Pd, as a chelate of divalent palladium.

8. A bath for electroless deposition of palladium as defined in claim 1 containing sodium hydrogen palladium ethylenediaminetetraacetate, C H N O NaPd, as a chelate of divalent palladium.

9. A bath for electroless deposition of palladium as defined in claim 1 containing potassium hydrogen palladium ethylenediaminetetraacetate, C H N o KPd, as a chelate of divalent palladium.

10. A bath for electroless deposition of palladium as defined in claim 1 containing ammonium hydrogen palladium ethylenediaminetetraacetate, C H N O Pd, as a chelate of divalent palladium.

11. A bath for electroless deposition of palladium as defined in claim 1 containing lithium hydrogen palladium ethylenediaminetetraacetate, C H N o LiPd, as a chelate of divalent palladium.

12. A bath for electroless deposition of palladium as defined in claim 1 containing disodium palladium ethylenediaminetetraacetate, C H N o Na Pd, as a chelate of divalent palladium.

13. A bath for electroless deposition of palladium as defined in claim 1 containing sodium potassium palladium ethylenediaminetetraacetate, C H N o NaKPd, as a chelate of divalent palladium.

14. A bath for electroless deposition of palladium as defined in-claim 1 containing sodium ammonium palladium ethylenediaminetetraacetate, C H N O NaPd, as a chelate of divalent palladium.

15. A bath for electroless deposition of palladium as defined in claim 1 containing sodium lithium palladium e'tliylenediaminetetraacetate, C H N o NaLiPd, as a chelate of divalent palladium.

16. A bath for electroless deposition of palladium as defined in claim 1 containing dipotassium palladium ethylenediaminetetraacetate, C H N O Pd, as a chelate of divalent palladium.

17. A bath for electroless deposition of palladium as defined in claim 1 containing potassium ammonium palladium ethylenediaminetetraacetate, CloHlfiNsOBKPd, as a chelate of divalent palladium.

18. A bath for electroless deposition of palladium as defined in claim 1 containing potassium lithium palladium ethylenediaminetetraacetate, C H N O KLiPd, as a chelate of divalent palladium.

19. A bath for electroless deposition of palladium as References Cited defined in claim 1 containing dilithium palladium ethyl- UNITED STATES PATENTS enediaminetetraacetate, C H N O Li Pd, as a chelate of 2,8723 46 2/1959 Miner XR divalent Palladium 2,915,406 12/1959 Rhoda et a1. 106'1 20. A bath for electroless deposition of palladium as 0 2 91 401 12 1959 Puls at XR defined in claim 1 containing lithium ammonium palladium ethylenediaminetetraacetate, C H N O LiPd, as a OTHER REFERENCES chelate of divalent palladium. Brenner, A.: Metal Finishing, Electroless Plating 21. A bath for electroless deposition of palladium as 10 Comes of Age, vol. 2, No. 12, December 1954, pp. 61- defined in claim 1 containing diammonium palladium ethylenediamlnetetraacetate, C H N O Pd, as a chelate JULIUS FROME, Primary Examinerof divalent palladium.

22. A bath for electroless deposition of palladium as HAYES: Asslstant Examine"- defined in claim 1 containing calcium palladium ethylene- U.S. Cl. X.R. diaminetetraacetate, cmH 'N O caPd, as a chelate 0f 124 130 divalent palladium.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,418,143 December 24, 1968 Alexander Sergienko It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 65, "C H N O Pd should read C H N O K Pd Signed and sealed this 24th day of March 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

